Technical Abstract:
Ammonium and nitrate are the major forms of N present in tropical soils. However, in plantation crops a substantial amount of N is present in ammonium form. A climatically-controlled growth chamber was used to assess the influence of forms (NO3, NH4, and a mix of NO3 + NH4) and levels (1.5 to 12.0 mM) of N on the growth and concentration of the macro-nutrients of cacao. Plants were grown in one gallon containers of vermiculite and exposed for 14 hours to 350 umoles s-1 m-2 light intensity at 30° C and 75% RH. Containers were surface irrigated once daily with 100 ml of N source and level, using a modified Snyder nutrient solution, for five days, washed with water on the sixth day, and let dry on the seventh day. This regime was repeated until plants were harvested at 57 days of growth. At harvest, growth parameters (shoot and root dry weight, shoot height, leaf area, and root length) were measured, shoot samples were ground and digested in acid mixture, and elemental composition was determined by the atomic absorption spectrophotometry. N composition was determined by a LECOCHN analyzer. Growth parameters and macro-nutrient concentration of cacao varied depending on the form and level of N. Increasing NO3-N from 1.5 to 6.0 mM and NH4-N from 1.5 to 9 mM increased the growth parameters. In mixed N (NO3 + NH4) treatment, growth parameters increased with increasing N up to 3.0 mM NO3-N. However, there was a decline in growth at 6.0 mM NO3, but further increases in N increased the growth parameters. Irrespective of forms of N, increasing levels of N increased the concentration of N and S. Forms and levels of N had no effect on concentrations of Ca and Na. Concentrations of K and Mg increased with increasing levels of NO3-N and decreased with increasing levels of NH4 and Mixed N. Forms and levels of N had no effect on P concentration. However P concentrations were higher for NO3-N and mixed N (NO3 + NH4) treatments. Based on these results, it appears that higher levels of N, irrespective of its form, are toxic to cacao.